Powder coatings have gained considerable popularity in recent years over liquid coatings for a number of reasons. Powder coatings are virtually free of harmful fugitive organic solvents and, therefore, give off little, if any, volatiles during curing. This eliminates solvent emission problems and creates a healthier environment for workers employed in the coating operations. Powder coatings also improve working hygiene, as they are in dry solid form and have no messy liquids associated with them which adhere to workers' clothes and coating equipment. They are relatively non-toxic as well and can easily be swept up in the event of a spill without requiring special cleaning and spill containment supplies. Another benefit is that they are virtually 100% recyclable. The oversprayed powders are normally reclaimed and fed back into the original powder feed during the coating operation, leading to high coating efficiencies and minimal waste. Yet, despite such advantages, traditional thermosetting powder coatings have not been suited for coating heat sensitive substrates, since the temperatures at which these powders must be cured are usually higher than the heat sensitive substrate can withstand.
With the increased desire to coat heat sensitive parts with powder coatings and realize the foregoing benefits, recent emphasis has been placed on developing powders that permit polymerization or curing at lower temperatures. One class of low temperature-cure powder coatings recently developed for heat sensitive substrates are UV curable powder coatings. Such UV curable powders can be formulated to melt-flow and cure and produce desired smooth glossy coatings at much lower temperatures than had ever been possible with traditional thermosetting chemistry, which is primarily due to the curing reaction being initiated solely by ultraviolet radiation rather than heat. The UV curing mechanism also enables production of the powders in traditional melt-blending equipment and storage at room temperature without triggering unwanted prereaction.
UV curable powders are typically prepared from solid unsaturated non-crystalline base resins, solid unsaturated non-crystalline crosslinker resins, solid photoinitiators, flow additives, other performance-enhancing additives, and optional pigments and inert fillers. It is also common to replace some or all of either the base or crosslinker resin with a crystalline material, as taught, for example, in EP 0 636 669 to DSM. One UV powder coating promoted by DSM that is presently preferred employs a blend of a non-crystalline base resin and a co-reactive crystalline crosslinker resin. Specifically, it comprises a stoichiometric blend of a solid, relatively polar, unsaturated amorphous (non-crystalline) polyester base resin with fumarate or maleate unsaturations and a glass transition temperature (Tg) of about 125.degree. F. (the Tg of the base resin being sufficiently high for desired blocking resistance but still low enough for desired low temperature melt-flow behavior), and a solid, somewhat incompatible, relatively non-polar, crystalline divinyl ether urethane crosslinker resin with a melting point (Tm) of about 195-230.degree. F. (the Tm of the crystalline resin being above the Tg of the base resin and also above traditional melt-processing temperatures to avoid destruction of intact crystal structures during processing and attendant loss in melt-processing efficiency and blocking resistance), together with a solid photoinitiator, flow control agent, and optional pigments. The presence of crystalline ingredients, in particular, has been found highly desired because the powders will exhibit low melt viscosity and excellent flow out behavior during the initial melting stage of the coating process, allowing the powders to readily coalesce into smooth molten coating films which upon subsequent UV curing and develop into exceptionally smooth coatings with desired glossy appearance.
However, one drawback with the use of crystalline materials, particularly crystalline crosslinker resins, having such high melting points (Tm) is that in order to obtain cured coatings with desired smooth glossy appearances, the coating must be UV cured at temperatures above the melting point of the crystalline component. If the temperature of the molten coating after flow out is allowed to drop below the melting point to the point at which the crystalline component visually appears to recrystallize in the coating prior to UV curing, which can happen during transfer of the coated part from the melting to UV curing operation and/or as a result of the coated part having a variable mass, the cured coating will have an undesirable haze or blush on the surface with attendant loss in gloss and smoothness. The haze is believed to result as the crystalline resin component separates out and migrates to the surface of the coating whereupon it is believed to recrystallize back into intact crystal structures. This imparts to the coating a cloudy, rough, matte appearance characteristic of crystalline resins. While this is desirable when making low gloss coatings, it becomes troublesome when trying to control hazing and obtain higher gloss films at low temperatures. Moreover, with increased demand to coat heat sensitive parts that can only withstand temperatures near the melting point of the crystalline ingredient, there is a need to provide a non-hazing UV curable powder coating that can be cured at such low temperatures and still produce a uniform haze-free smooth glossy finish over the part.
The limitations of traditional UV curable powders containing a crystalline resin can be best seen when trying to coat assembled electric motor casings which house a number of heat sensitive components, including working parts and electric circuitry associated therewith. To avoid damaging such components, the melt-flow and curing temperatures must be kept at levels that are only slightly above the recrystallization point. To further complicate the matter, these motor casings tend to have a variable mass. During coating, as the powder is melted at the desired conditions for curing, the heavier mass sections create heat sinks and cause the temperature of the molten coating in these sections to drop below the point at which the crystalline ingredient recrystallizes, while in other sections the coating temperature is kept above the recrystallization point. As a result, the cured coatings produced therefrom have an unacceptable mottled appearance due to the haze appearing around the heavier mass sections, rather than having a consistent smooth glossy coating across the part.
To overcome the hazing and mottling problem, it is possible to use non-crystalline resins alone which do not recrystallize. However, in order to achieve adequate flow out at the low temperatures suited for heat sensitive substrates, the Tg of the non-crystalline resins must be lowered considerably, which would render the powders physically unstable and susceptible to blocking or sintering during storage at room temperature. Powders that block are extremely difficult to meter and spray during the coating operations and lead to inconsistent spray patterns and defects in the coatings. Reducing the amount of crystalline resin, particularly the crystalline crosslinker resin, to levels at which recrystallization does not occur (i.e., below 10 wt. % of the resin system) has also been tried, but at such low levels the powders exhibit poor flow out at low temperatures and produce incompletely cured coatings with severely textured appearances.
It would be desirable to provide an improved UV curable powder coating composition containing crystalline resins adapted to prevent hazing in the coating formed therefrom when cured at low temperatures.